<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>写一个hello world</title>
</head>
<body>
    <script type="text/javascript">
    var oP=document.createElement('p');
    var body=document.getElementsByTagName('body');
    oP.innerHTML="Hello world";
    body[0].appendChild(oP);

    </script>
</body>
</html>